A number of systems have been developed to drive rotation of the propeller shaft or shafts of a sea-going vessel. For example, a steam turbine, gas turbine, combustion engine, electric motor or the like may be used to drive rotation of the shaft, either directly or via a reduction gearing arrangement.
During operation, the shaft can often become hot and may be subject to a degree of expansion and it has been found that, when rotation of the shaft is stopped, the static shaft may be susceptible to distortion in the form of sagging, bowing or other damaging temperature effects. In order to overcome or mitigate damage to the shaft, turning gear may be employed to provide continuous, relatively slow rotation of the shaft when the turbine or other drive is not in operation; continuous rotation of the shaft assisting in preventing shaft distortion.
The turning gear may also be used to rotate the shaft from rest, thereby reducing the start-up torque required to initially rotate the shaft prior to engagement of the turbine or other drive.
Furthermore, the turning gear may be used to hold the shaft stationary in order to facilitate repair or maintenance of the shaft as required.
It will be recognised that significant loads may be transmitted through the turning gear and the shaft and, for example, with regard to larger vessels, it has been found that reaction loads generated in the turning gear mechanism due to shock loading can result in damage to the turning gear.